The presence of large deposits of oil shale in the semi-arid, high plateau region of the western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. The term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products, including hydrocarbon products. It is the formation containing kerogen that is called "oil shale" herein; the carbonaceous liquid product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either mining the kerogen-bearing shale and processing the shale on the surface or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the spent shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes. According to both of these approaches, oil shale is retorted by heating the oil shale to a sufficient temperature to decompose kerogen and produce shale oil which drains from the rock. The retorted shale, after kerogen decomposition, contains substantial amounts of residual carbonaceous material which can be burned to supply heat for retorting.
One technique for recovering shale oil includes forming an in situ oil shale retort in a subterranean formation containing oil shale. At least a portion of the formation within the boundaries of the in situ oil shale retort is explosively expanded toward one or more voids excavated in the subterranean formation to form a fragmented permeable mass of formation particles containing oil shale in the retort. The fragmented mass of particles is ignited near the top of the retort to establish a combustion zone. An oxygen-supplying gas is introduced into the top of the retort to sustain the combustion zone and cause it to move downwardly through the fragmented mass. As burning proceeds, the heat of combustion is transferred to the fragmented mass of particles below the combustion zone to release shale oil and gaseous products therefrom in a retorting zone. The retorting zone moves from the top to the bottom of the retort ahead of the combustion zone and the resulting shale oil and gaseous products pass to the bottom of the retort for collection and removal. Recovery of liquid and gaseous products from oil shale deposits is described in greater detail in U.S. Pat. No. 3,661,423 to Donald E. Garrett.
As used herein, the term "retorting zone" refers to that portion of the retort where kerogen in oil shale is being decomposed to liquid and gaseous products, leaving residual carbonaceous material in the retorted oil shale. The term "combustion zone" refers to a portion of the retort where the greater part of the oxygen in the retort inlet mixture that reacts with the residual carbonaceous material in the retorted oil shale is consumed.
It has been found desirable in some embodiments to have an intact subterranean base of operation above the fragmented permeable mass of formation particles in an in situ oil shale retort. Such a base of operation facilitates the drilling of blastholes into underlying formation for forming the fragmented mass in the retort and facilitates ignition over the entire top portion of the fragmented mass. Additionally, having a base of operation above the fragmented mass permits control of introduction of oxygen-supplying gas into the retort, provides a location for testing properties of the fragmented mass, and provides a location for evaluation and controlling performance of the retort during operation.
The base of operation is separated from the retort by a layer of unfragmented formation extending between the top boundary of the retort and the floor of such a base of operation. The layer of unfragmented formation is termed a "sill pillar" which acts as a barrier between the in situ oil shale retort and the base of operation during retorting operations. It is, therefore, important that the sill pillar remain structurally sound, both for supporting the base of operation and for preventing entry of heat and gases into the base of operation during the retorting process.
Techniques for forming an in situ oil shale retort containing a fragmented permeable mass of formation particles and having a sill pillar of unfragmented formation between the top of the fragmented mass and an overlying base of operation are described in U.S. Pat. No. 4,118,071 by Ned M. Hutchins and in U.S. Pat. No. 4,192,554 by Thomas E. Ricketts. U.S. Pat. Nos. 4,118,071 and 4,192,554 are incorporated herein by this reference.
In retorts where no base of operation is provided, the formation overlying the fragmented permeable mass of formation particles extends all the way to the ground surface. In such an embodiment, blastholes are drilled through the overlying formation and ignition of the fragmented mass of particles is accomplished from the ground surface.
Examples of other techniques used for forming in situ oil shale retorts are described in U.S. Pat. No. 4,043,595 by French; U.S. Pat. No. 4,043,596 by Ridley; U.S. Pat. No. 4,043,597 by French; and U.S. Pat. No. 4,043,598 by French et al, each of which is incorporated herein by this reference.
When unfragmented formation is explosively fragmented and expanded, e.g., toward a void space when forming a retort, it increases in bulk due to void spaces in interstices between the particles. The maximum expansion of an oil shale formation into an unlimited void results in a fragmented mass of oil shale particles having an average void fraction of about 35 percent; that is, about 35 percent of the total volume occupied by the fragmented mass is void space between the particles. The volume occupied by the fragmented mass is about 55 percent larger than the volume occupied by the original unfragmented formation after such unlimited or free expansion.
A "limited void" is one where the void space available for explosive expansion is less than needed for free bulking of the formation expanded toward that void. Thus, if a void has an excavated volume less than about 35 percent of the total of the volume of the void plus the volume occupied by formation explosively expanded, it is necessarily a limited void. It has been found that factors in addition to total available void can make a void "limited" even though the total available void may appear sufficient for free bulking.
When oil shale is explosively expanded toward a limited void, the void fraction of the fragmented mass of particles formed can be no more than permitted by the available void space of the void and, in some instances, has been found to be less. It is believed that the void fraction of the fragmented mass can be less than the available void space provided by such a limited void because when oil shale is explosively expanded toward the void, gases from the detonation may not have full opportunity to act on the oil shale particles before such particles reach obstructions, such as adjacent walls, a face opposite to the expanding formation, or oil shale expanding from the opposite sides of the void.
Thus, when formation is expanded toward one or more limited voids for forming an in situ oil shale retort, a void space can remain in the completed retort between the surface of the fragmented mass of particles formed and overlying unfragmented formation, i.e., the retort top boundary.
When a void space is between overlying unfragmented formation and the fragmented mass in a retort, heating of the overlying formation during ignition and/or retorting operations can result in spalling or sloughing of formation into the retort. This can prolong the ignition process and, in some instances, can make ignition impossible. Additionally, sloughed formation can be heated sufficiently to consume at least a portion of the oxygen being supplied to the retort during retorting operations. This can upset the desired material balance in the retort and deleteriously affect the amount of products recovered.
When the retort has an overlying sill pillar and a void space exists between the top of the fragmented mass in the retort and the bottom of the sill pillar, i.e., the top boundary of the retort, sloughing of formation from the bottom of the sill pillar can weaken it. If the sloughing is sufficient, the sill pillar can lose its structural integrity, thus rendering the base of operation unsafe for occupancy.
It can, therefore, be important to provide support for unfragmented formation which comprises the top boundary of an in situ retort to reduce or eliminate sloughing of formation from the top boundary into the retort.